Commercially available stand-up plastic storage bags for storing food products and especially for storing milk with freezer capability include bags having an expandable bottom gusset. This gusset allows the bag to stand upright in preparation for filling, when filled, when stored in a freezer; while also having a flat conformation when empty so that it can be stored flat and in an efficient stack when empty.
U.S. Pat. No. 4,837,849 shows a stand-up plastic storage bag which has three layers including an exterior preferably nylon layer, an interior preferably polyethylene layer, and a glue layer therebetween. The nylon layer is stated to have a high modulus and high heat distortion resistance to improve hot liquid stand-up stability.
U.S. Pat. No. 8,303,182 shows a plastic bag with a bottom reinforced by lateral ridges.